Validation refactoring

[ashklianko]

1. Potential error in Form.ts where single fieldset validation state change by some reason declares that entire form validation state is the same as in the fieldset

fieldset.onValidityChanged((event) => {
     this.notifyValidityChanged(event.isValid());
});

2. Form, FormView, InputTypeView (potentially more classes) apply different approaches to validation: some return validation result, some don't.
3. Develop common approach to validation and api for it (we definately missing isValid() method)

To be continued after further code inspection